Get desktop bounds correctly on X11

[ajerick]

Partially fix aseprite issue 3118. X11 was never made with multi-monitor support in mind and, as a result, finding the workarea on multi-monitor setups is impossible without awful hacks. Instead, use the Xrandr X11 extension to query monitors, and their properties, to replace X11's display property in the operating-system level "Screen" implementation.

With this patch, screen bounds will be correct on multi-monitor setups in X11 however the illusive workarea property will mirror the bounds.

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